In a standard suction/blowing mold system for producing complex three-dimensional hollow articles, typically of plastic, a hollow preform having a closed leading end is produced by an extruding apparatus. This leading end is engaged in an upper intake end of a die cavity that is formed as a passage between two fitted-together female die halves and that is of somewhat greater cross-sectional size than the preform. Suction is applied to the opposite lower or output end of the die passage to draw the preform down through the mold as it is either emitted by the extruding apparatus or advanced by a manipulator.
Once the preform is threaded completely through the mold cavity, its leading and trailing ends are pinched closed at the upper and lower passage ends. A needle is then inserted centrally into the preform and gas is blown into it to expand it to fill the mold cavity. Once the material of the preform has cured, the mold is opened and the finished workpiece is removed. It will have the exact shape of the mold passage with no flashing or sprues. In particular its inner surface will be perfectly smooth and without seams. Such a system as described in “Technische Mitteilungen Krupp” (February 1998/pages 49/50) is used, for instance, to make complexly shaped manifold or feed tubes for appliances.
In drawing the soft preform through the mold passage, which is typically nonstraight as it is just this type of application that is particularly suited for suction/blow molding, it is necessary in the interest of speedy production to apply enough suction to move the preform as rapidly as possible while not at the same time applying so much suction that the preform is torn or damaged. As the leading end of the preform moves along straight portions of the mold cavity the suction can be great for a rapid travel speed, but when corners are being rounded a smaller pressure differential is needed to slow down travel and prevent damage to the preform.
In the 2 sec the entire operation takes it is not possible to significantly vary the throughput rates of the large-capacity blowers or pumps used to produce the necessary suction. Thus the known system must be set to use at many times lower suction that what could actually be used.